An auxiliary power unit (APU) system provides auxiliary and/or emergency power to aircraft electrical loads. In conventional APU systems, a dedicated starter motor is operated during a starting sequence to bring a gas turbine engine up to self-sustaining speed, following which the engine is accelerated to operating speed. Once this condition is reached, a generator is coupled to and driven by the gas turbine engine during operation and the generator, in turn, develops electrical power for the electrical loads.
In addition to supplying electrical power to the aircraft, APUs commonly supply pneumatic power to a wide variety of aircraft accessory devices and systems. Accessory system pneumatic power is typically provided by forcing bleed air from the APU compressor section through a main bleed duct to the accessory system's supply. In order to prevent surge of the APU compressor used to power the pneumatic accessory system, it is necessary to maintain a certain minimum flow rate through the main bleed duct regardless of the requirements of the accessory system(s). Surge is a phenomenon that is caused by the detachment of fluid streams along compressor airfoils, which leads to instability in the flow and thereby in the operation of the compressor.
APU-supplied accessory pneumatic systems normally have widely fluctuating compressed air requirements and are automatically controlled to correspondingly regulate the amount of bleed air they receive from the compressor by modulating a bleed air valve. To accommodate a decrease in accessory air demand, and maintain the compressor through flow above its surge level, a surge bleed duct is typically connected to the main bleed duct to provide an alternate outlet flow path for the compressor bleed air as the compressed air requirements of the accessory system decrease or cease altogether. Flow through the surge bleed duct is regulated by modulating an anti-surge valve separate from the main bleed air valve.
There is a continuing need in APU design to reduce cost and complexity by reducing the total part count in the APU system.